Michael reactions in which base-catalyzed addition of carboanions to activated unsaturated systems are utilized in the crosslinking of resinous compositions for use in coatings. These compositions are advantageous in that the Michael reaction gives a chemically stable crosslinking bond without forming any reaction by-product.
Typical examples of prior art include the following patent documents:
U.S. Pat. No. 4,408,018; PA1 U.S. Pat. No. 5,017,649; PA1 U.S. Pat. No. 5,084,536; PA1 EP-A-0448154 PA1 JP-A-01/121341 (EP-A-0310011); PA1 JP-A-01/204919 (EP-A-0326723); PA1 JP-A-01/152117; PA1 JP-A-01/167355; PA1 JP-A-01/167356; and PA1 JP-A-01/229019. PA1 (a) a component containing a plurality of a .alpha., .beta.-ethylenically unsaturated carbonyl functions in the molecule; PA1 (b) a component containing a plurality of at least one active hydrogen-containing groups selected from the group consisting of an activated methylene group, an activated methyne group and an hydroxyl group; PA1 (c) an onium salt selected from the group consisting of a quaternary ammonium salt, a quaternary phosphonium salt and a tertiary sulfonium salt; PA1 (d) an epoxy compound independently added to the composition or covalently attached to at least one of the above components (a) and (b).
Michael reactions in general require a base catalyst for forming carboanions from active hydrogen sites such as activated methylene or methyne groups. Most of the prior art compositions utilizing the Michael reaction for crosslinking employ a strong base such as alkali metal hydroxides, alkali metal alkoxides, quaternary ammonium hydroxides or carbonates, tertiary amines, guanidines, amidines and tertiary phosphines.
Because such strong base catalysts may catalyze the Michael reaction even at ambient temperature, systems containing the strong base generally have to be formulated into a two component composition. The strong base catalyst may also hydrolyze or otherwise deteriorate resin components when retained in the films applied on substrates. For example, amines may cause yellowing of the cured films. Alkali metal based catalysts are hardly soluble in organic solvents conventionally used in coating compositions and, therefore, may result, when used, in cured films of unsatisfactory appearance.
JP-A-01/167356 discloses the use of quaternary ammonium salts with carboxylic acids. However, the quaternary ammonium carboxylates themselves have only a weak catalytic activity and do not promote the crosslinking reaction satisfactorily when used alone. Halides and other ammonium salts have been proven to have no or little catalytic activity when used alone.
JP-A-01/204919 discloses a catalyst system containing a tertiary amine and an epoxide. In the curing mechanism of this system, unlike the reaction in the absence of an activated methylene, the tertiary amine reacts first with the epoxide and then with a weakly acidic methylene component to form a quaternary ammonium salt resulting in the activation of the methylene group susceptible to the reaction with an alkene component. This means that carboanions are formed by the withdrawal of a hydrogen atom from the activated methylene as the tertiary amine is quaternized with the epoxide. It will be easily appreciated that the above activation mechanism does not apply to a pre-formed quaternary ammonium salt.
The present invention has its basis in the discovery that pre-formed quaternary ammonium or like onium salts having no or little catalytic activity may exhibit an adequate activity on a Michael type crosslinking reaction in the presence of an epoxide and various shortcomings of the prior art catalyst systems may be overcome by the present invention.